Position determining system



Aug. 6, 1946. s. w. sEELl-:Y 2,495,238

POSITION DETERMIING SYSTEM Filed April 1s, 1940 e sheets-sheet 1 Stuart l/V.' Seeleyl Aug. 6, 1946. s. w. s'EELEY POSITION DETERMINING SYSTEM Filed April 13, 1940 6 Sheets-Sheet 2 Cttorncg Aug. 6, 1946. s. w. sEELEY POSITION DETERMINING SYSTEM Filed April 13, 1940 6 Sheets-Sheet 3 9 *TM wr E# .M0 5T @w aar/wf@ aufn/7*@ Snventor @FFME/107670@ Itorucg Alg- 6, 1946 s. w. sEELEY 2,405,238

POSITION DETERMINING SYSTEM Filed April 15, 19470 6 Sheets-Shea?l 4 PULSE 'SELL' C 710)? PLM'L' x/oLMc-l.' U

OUTPUT 0 4 Snventor l/ Stuart l/Keeley (Ittorneg Aug. 6. 1946. s. w. sEELEY 2,405,233

POSITION DETERMINING SYSTEM Filed April 15, 1940 6 Sheets-Sheet 5 I l I Y Vl Y Y I z/PPf/P 7055 FG. L?.

VOA T1965 Y Imventor Pfl/75E @E VfPsEA UP/2:56

't Lari VIZ-Seeley Gttomcg Allg- 6, 1946. s. w. SEELEY 2,405,238

PsITloN DETERMINING SYSTEM Filed April 13, 1940 6 Sheets-Sheet 6 U E Z a W fmnwa. W w WMM s M60/711s 1.H w j W M B7 J m a w 5 Patented Aug. 6, 1946 2,405,238 POSITION DETEBMINING SYSTEM Stuart W. Seeley, Roslyn, N. Y., assignor to Radio Corporation of America, a. corporation of Dela- Ware " Application Aprilia, 1940, serial NBL 329.434

1s claims. (ci. 25o-1) This invention relates to a system and method for accurately determining the instantaneous position of a movableobject with respect to one or more reference points whose locations are known. More particularly, it relates to a radio control system whereby a movable object may be guided directly to the location of a predetermined objective.

By the term movable object is herein meant any aircraft, ship, submarine, motor vehicle or the like. The invention is particularly useful for directing the night of an airplane to a position directly above a predetermined objective such as an airport, city, crossroad, bridge, or the like, and such a particular application will be herein described, although it is to be understood that the invention is nowise limited to the particular arrangement described.

Aircraft position locating systems are known in which directional bearings of two or more ground stations `are successively, or simultaneously taken from an airplane. By drawing lines on a map having angles which correspond to the bearings of known ground stations and which pass through the points on the map corresponding to the location of the ground stations, the position of the airplane may be determined by the point at which the lines intersect. Systems are also known by means of which such bearingsA are automatically taken and recorded so as to provide a continuous indication of the location of the airplane with respect to predetermined ground stations. Such directional bearings, however, are subject to substantial errors due to night effect and other causes, as is well known. It is the principal object of this invention to provide a position determining system for movable objects Which is free from the errors of the known systems and which is sufficiently accurate to indicate the location of the object with respect to a given point or points within an error of a few hundred feet, even where the reference points or ground stations are several hundred miles distant. Another object of this invention is to accomplish the foregoing result in such a manner as to provide the navigator, or an aircraft, for example. with a continuous visual or semipictorial indication of his distance from the known control points, and from a third point which is to be the objective of his flight.

A further object of this invention is to accomplish the foregoing results in such a manner that the system is substantially free from interference due to static and other electrical disturbances, and likewise substantially free from inter- 2 ference by other transmissions. At the same time, the nature of the transmission is such that it preserves substantially complete secrecy, thus making it impossible for unauthorized listeners to utilize any signals radiated from the airplane or the control station which may be intercepted.

A further object of the present invention is to provide a system which is not dependent `for its operation upon the relative amplitudes of the transmitted or received signals but merely upon the presence or absence of a signal. So far as is known most of the systems of the prior art operate in conjunction with overlapping directional ilelds which are alternately keyed, the on course indication being a function of the relative amplitudes of the two iields. T'he position indicators of the prior art generally utilize directional loop antennas for the successive determination of the bearing of the airplane with respect to two or more ground stations. The present system differs from the known prior art in that it depends for its operation upon the invariable velocity of propagation of radiant energy. As a result, the accuracy of the present system exceeds that of all known prior systems.

A still further object of this invention is to provide a position determining system for airplanes and the like, in which the distance from the airplane to two or more ground stations is accurately determined, as distinguished from known systems in which the angular bearing of the ground'station is determined.

A still further object of this invention is to provide a visual indicator for airplanes, and the like, which may be preset to guide the plane to a predetermined objective, the indicator being provided with an objective index and one or more variable indexes which correspond to the position of the plane at any time. 'I'he airplane is then guided along a path which causes the variable index or indexes to approach the objective index, and finally to coincide therewith. At such time the pilot of the airplane knows that he has reached his objective and may then prepare to land or accomplish any other mission which has been assigned to him.

A still further object of this invention is to provide a simple, adjacent, easily installed, and substantially fool-proof form of guiding or position nding system by means of which it is possible to locate the position of an aircraft in space in a plane which is either parallel to or normal to the surface of the earth, as well as to determine the instantaneous location 'in space of an airselected objective.

trol station and the secondarycontrol station.

The locations of these control stations are. fixed relative to each other and are accuratelyknown. 'I'he control stations may be located at permanent installations, or they may be located vin trucks orv -4 splitter; Figure 9 is the circuit diagram of a Duke selector; Figure 10 is a diagram illustrating the operation of the step selector of Fig. 9; Figure 11 is the circui-t diagram of a clipper tube; Figure 12 is the circuit diagram of a keying tube; Figure 13 .is a diagram illustrating the' input' and output voltages of the keying tube; Figure 14 is the cirother vehicles so as'to be movable to any locationsV as conditions change. V'lihe control stations may evenbe located in balloons suspended above the earth at predetermined locations and heights, or

the control stations may be located in ships at sea whose positions are accuratelyA known. Still i'urv ther the control' stations may -even be located' beneath the surface of the earth r beneath the water, or any combination of the above may be used. VIt is to be understood, however. the locations of the control stations remain ilxed during any given night. Y

The equipment which is utilized at each control station includes means for reradiating a pulse of radio frequency energy. The term "reradia is intended to include reflection as well as actual reception and retransmission. The preferred arrangement consists of a receiver and a transmitter. The receiver operates to receive the radio frequency transmissions from the guided aircraft. It is connected to the transmitter, and modulates the same, which reradiates the received wave at a slightly different radio frequency. In order to distinguish one ground station from. another the transmitter frequencies at each station are difl ferent. The equipment required on the movable object includes a pair of receivers responsive to the ground station transmitters and a transmitter of the impulse type which is well known in the art. Such a transmitter radiates an extremely cuit diagram of a phase reverser and clipper; Figure 15 -is the circuit diagram of a vtime delay network; Figure 16 is the circuit diagram o1' a commutatorgand Figure 17 is the circuit diagram of an impulse transmitter;

VThe present invention has a wide variety of applications, other than the embodiment herein illustrated. It should be clearly understood, therefore, that anyassumptions made are only for the purpose of illustration and are not intendedl in any manner to limit the -scope of the present invention. Referring vto Fig. 1, the viewing screen 2| of a cathode ray tube is illustrated. By suitable apparatus which will be hereinafter described, a series of ten horizontal lines are traced acrossthe face of the screen by suitably deilecting the electron beam. The general nature ofthe beam deflection is such that it is caused to Vmove horizontally from lef-t to right at a uniform rate in a given time and then to return lfrom right to left in a very much shorterv time to begin short pulse of radio frequency energy,the duration of the pulse being considerably less than the y time required for the wave of radiant energy to travel from the aircraft to the ground station and return to the aircraft. By measuring Vthe time required for each impulse to be radiatedi'rom the tracing the second horizontal line in a similar manner. In each case the return time vof the beam is made as rapid as possible, and the time required for returning the beam to its starting position is usually of theorder of 2 percent of the time required to trace one line. In this manner the beam successively traces the tenhorizontal lines moving in each case from left to right in a time, which for the sake of illustration, may be assumed to be $50.000 of a second. At the same time the beam=deflects vertically, in steps or linearly with respect to time, so as to move the beam from the rstline to the tenth line -in a period approximately equal to 1,4000 of a second. As before, the beam is returned from the bottom Aof the screen to its initial position very rapidly so airplane and received again, the actual .distance i of the aircraft from each of the control'stations may be determined. It will be appreciated that this information is all that is necessary -to d etei'- mine the position of the aircraft-,with to the two ground stations. l In'the case Voil* theindi- Y cator system herein described, it is not necessary for the pilot to make any c alculationsin ilying to a predetermined objective, while at thesam'e time, his distance from the control stations'and from the objective is continuously indi- This invention be better understood from that substantially no timeis' lost between the completion ofthe tenth line and the start of the firstV line.

It will be appreciated that the beam traces ten lcomplete lines, or one frame at the rate of 3,000

frames persecond. Since a radio wave is known to travel at the rate of 300,000,000 meters per second, it will be appreciated that a pulse of radio energy will travel a distance of 10,000 meters througlispace during the time required for the beam to trace each horizontal line. If it now be assumed that the several line traces produced by the cathode ray beam upon the luminescent viewthe following description when considered in` conscope is indicated by the appended claims.

Referring to the drawings, Figure 1 is a sketch indicating the cathode ray indicator located .in the movable object; Figure 1a is a sketch similar to Figure 1; Figure 2 is a sketch illustrating the tiator; Figure 7 is the circuit of a saw-tooth discharge tube; Figure 8 is the circuit of avphase nection with the accompanying' drawingsand its ing target 2l o f the `cathode ray tube cover an area of approximately 4 inches square, it will at once become evident that during the time the cathodel ray beam has moved along one line of the screen a distance of 1 inch, a radio wave will travel approximately 2500 meters through space.

It is realized that this ligure is only approximate, since it does not take into consideration the small period of time required for the return of the cathode ray beam. In order to simplify the explanation, however, this return time will be neglected, but -it is to be understood that actual calculations utilizing this instrument must include a consideration of the return time.

In accordance with the basic conception of this invention a short pulse of radio energy is transmitted from the aircraft at a time To. This im- Pllliie is utilized to initiate the scanning cycle of the cathode ray beam. Thus, the left hand end -of the ilrst scanning line corresponds to the time To at which the first impulse is radiated. Referring now to Figs. 1a and 2, it will be seen that the impulse travels from the aircraft 23, is received by the receiver 25 at the primary control station, reradiated from the transmitter 21 and received back at the aircraft on a receiver 29. The received impulse is then applied to the cath'- ode ray screen so as to produce a small upward deiiection P in the scanning beam or to vary its intensity in accordance with well known practice. The elapsed time is, of course, a measure of the total distance through which the impulse has traveled, plus some fixed time intervals which are a function of the circuit characteristics of the receivers and transmitters utilized and which may be accurately predetermined. During this elapsed time the cathode ray beam has traversed a number of horizontal line traces and is, for example, traversing the eighth line at the instant the received impulse delects the cathode ray beam to form a position index mark P.

At the Same time the transmitted impulse will be received by the receiver 3| of the secondary control station, reradiated on a diierent frequency by transmitter 33 and received again by the receiver 35 which is located on the aircraft. 'I'he resultant impulse is also utilized to deflect the cathode ray beam in a vertical direction, but in order to distinguish this impulse from the one previously received it may be phased so that it deects the beam downwardly to produce the position index S illustratedin Fig. la. It has been assumed in this instance that the aircraft is at the particular moment slightly nearer the secondary control station than the primary control station, so that the position index S from the latter station is received before the position index P has been received from the primary control. station.

It thus becomes apparent that the distance of the aircraft from the two control stations may be determined by measuring the position of the impulses P and S along the scanning lines. Specific objects of this invention, however, make possible a far more accurate determination than would be possible by the suggested measurement, and in addition, a system is provided by means of which the pilot may fly to a predetermined objective without having to actually measure the position of the received position index pulses along the scanning lines, or to make any calculations whatsoever. In accordance with this modification of the invention a pair of objective indexes OP and OS are provided which represent the destination of the aircraft. That is, the pilot is merely required to ny the aircraft until the variable position indexes P and S coincide with the respective 'objective index, at which time he knows that he Vhas reached his objective and is within a few ,hundred yards of his destination.

Before the aircraft takes ofi for any given flight to reach a particular destination it is, of course, necessary to calculate the time distance of the objective from each of the given control stations. Now the time distance from the objective to the primary control station is not necessarily equal to the time distance from the objective to the secondary Hcontrol station. Nor is it desirable to provide two objective indexes corresponding to the two separate distances, since the pilot must then align the position index P with the objective index OP, and also align the position index S with the corresponding objective index OS. While such a system is entirely practical, it has the disadvantage that it is necessary to align two variable index marks with two ilxed index marks which may be and usually are located at diilerent points on the cathode ray screen. Accordingly, a preferred system is herein provided by means of which only one objective index is required even though the distance from the objective to the primary control station diers considerably from the distance to the secondary control station.

The preferred method of providing the xed objective index will now be described. It will be apparent that the time distance from the primary control station to the objective can readily be calculated, taking into consideration the known delays of the equipment. Assuming that the objective is further from the primary control station than from the secondary control station, the time distance from the primary control station is first determined. It is apparent that a mark could be placed over the scanning line corresponding to the calculated time distance. This would not produce a very accurate determination, however, since any change in the position of the lines would cause a corresponding error. It is desirable, therefore, to include the objective index in the scanning line itself by suitably defiecting the cathode ray beam to provide a mark at the instant corresponding to the precalculated time distance. One method of accomplishing this is to delay a, pulse initiated at the time the radiated pulse was transmitted by a period equal to the calculated time distance. This, however, would require a. time delay network of proportions which would be prohibitive in an aircraft. Consequently, provision is made for deriving a pulse which occurs a known interval after the transmission of the radiated pulse, the known interval being approximately the desired time distance. This approximate timing pulse may then be corrected and utilized to provide the objective index marker. Alternatively a series of fixed timing indexes may be provided, one of the timing indexes being selected as the objective index by suitable marking and the exact calibration eiected by shifting the time relation of the radiated impulse with respect to the timing impulses or vice versa. The latter arrangement is preferred and is described hereinafter.

Each line of Fig. l is seen to be divided into ve equal time periods by means of small index marks which occur at regular time intervals and which represent successive time periods of 1/isaouo second. These timing indexes not only make possible more accurate measurements along the ilne in case the system is to be used for the calculation of the distance of the aircraft from two control stations, but also are the source of the objective index which was referred to above.

In accordance with the preferred system, assuming the calculated time distance falls between the 38th and 39th timing index, in the 8th line, the timing index which is the rst to occur after the calculated time distance, i. e., number 39, is marked by means of a suitable mark O which may be placed upon the glass face of the cathode ray screen. In order to calibrate the device for distances intermediate the times represented by successive marking impulses, the transmitted impulse itself is then delayed by a calculated amount so that the received impulse coincides with the selected marking index O at the calculated time.

It now becomes necessary to consider the fact that the calculated time distance from the objective to the secondary control station is not necthe s that from the obiecuve to the primary control station. In this case a second impulse is radiated from the aircraft transmitter at a subsequent time which is so calculated 'that .itwill arrive back at .the receiver at y y the same'mstant that the erst transmitted impulse returns from the primary control station when-"the'aircraft is directlyover its objective.

-is'accomplishe'd by utilizing one of thein- `dexing impulses t'ogenerate a secondtransmitted` .l0 impulse ata known time interval after the trans-` 1 mission of the iirst'impulse to the .primary ycontrolstation. For example, the indexing impulse X appearing at thetime interval represented' by its locationV on the fourth scanning line is 'selected and utilized to release'an impulsefrom As before, corrections` the aircrafttransmitter. fortimeintervalsless than the period between to provide means for preventing thenrst impulse from deilecting the Acathode ray after retransmission by thesecondary control station and.

for preventing the subsequentimpulse i'romA deilecting the cathode ray'afterretransmission by 1 the `primary control station. Itthis werefnot 1 done, four impulseswouldbeobserved onthe cathode ray screen instead of two since the pulsey first to be transmitted tothe primaryjcontrolstation would als'obe` repeated 4back from thesecondary' control'station.- lllllewise, .-the` secondi transmitted would befrepeated back from both'. controlstations.. Consequently, it is .pro-

posed to synchronously commutatethe two. re-

vceivers and the circuit connecting ythe successive' modulating impulses to the transmitter at. a vrate which is relatively small with-respect to thetime being measured so" that one or more pulses are' sent vto andl received-from the primary `control y station andthen for 'a successive interval one or more pulses varesent to and received from lthe secondary control station.' 'I'herate of commutation, however, kisY preferably suilicient to v.provide persistence -of vision on vthe cathode ray*- screen so that both pulses are apparently simultaneously received. u i

-Before taking up the discussionoi' the particular apparatus and accomplishing the above described results, Vit is desirablertov consider-*means for extending the' range of the device beyond that which would be attained by the time interval rep- Since the resented by the ten scanning" lines. scanning pattern is repeated successively at a rate of 3,000` complete frames of ten lines each per second, it will be appreciated thatiI the dis.' tance from the aircraft to either control station is sufficiently great, i. e., in excess'of 100,000 meters, the retransmitted impulse will not :arrive during the rst 'scanning'peri'od` Ihat is, ifvthe .position index YP wereto appear on theflrst 'scan- 1 ning Vline this might indicate either that the. aircraft was verynear -the primaryv controlstation or at such a distance that the received impulse was not in fact on the first scanningline buton I the eleventh scanning line which coincides therewith. `In order to ascertain inwhich complete scanning frame the received impulse has been received asystem has been provided which per mits the selection of any one of ten complete 'I'hat is, when the aircraft is within a frames. certain .distance of the control station the instrumentwill be adjusted so that only the rst scan- Y ning frame will be applied to the cathode ray screemthe following nine scanning frames being suppressed.' This selecting mechanism is also provided with means for deinitely determining which one of the group of ten frames is being viewed'. Y

f The objective index0 may, for example, occur during'the fth scanning frame if the distance l 'from the primary control station to the objectifeis s uiciently great. The pilot, however, be-

ing' desirous Vof indicating his approximate distancev from the control station at all times, will first select `the first scanning trame and follow "theposition jindex P representing his distance from thezprimary control station along the var- 'ious'lines o1' the' rst scanning frame. As his distance increases the point P will move of! the 'v 'fright-hand -end voi! the tenth' line and will no longer appear until he operates the selector mechanismjand applies the second scanning frame .tothe cathode ray indicator. The variable index P will thenappear on the rst scanning line and'will move successively down to the tenth line as his distance from the primary control station increases. This process will then be continued until he reaches the general vicinity oi thev objective, at which time .the objective index VO in'the fifth scanning frame will appear.

Y Since it is desirable to simultaneously indicate fthe distance' of the aircraft from the secondary `'coi'itrolstation, it is preferable to follow a course which' lis generally equi-distant from the' two controllstations. The pilot will thus be able to ob- .'servehis relative position with respect to the two .fcontrolstations at all times. However, this will Y not always be the case, sinceas pointed out above,

y V separately timed pulses are radiated for the two i-:control stations to'compensate for-the difference in distance between the objective and the control stations. 'If 'the discrepancy is great enough, it

may-be necessary to alternately select one scanf-- ningl frame and then th'e other in order to check his relative'gdistance `from the two control stations. Alternatively during the initial part of the flight, it-may be desirable to'radiate but a single `pulse and'receive it from both control stations simultaneously 'inforder to provide a more accu- `i'ateindication of his generalposition with respect to the control stations. One switch is provided-forf'eifectively removing the commutator H5, and another rswitch is provided'ior disconnectingthe second modulating impulse from th'e transmitter.- Suitable switching arrangements tol vaccomplish this are fprovided. Upon approaclii'ng the objective the doubleimpulse system may thenvbe switched in so that the single objective.index, properly compensated for the relativeV distances to the two control stations, is

utilized. l

5A speciflcjembodiment ofthis invention will vnow be described by referring to lFig. 3 Awhich illustrates in'block diagramform the apparatus ljivhich'is 'installed in the aircraft to accomplish the above-described results.

. Reference numeral`36 indicates an oscillator, the frequency of-which is accurately maintained vat a `constantvalue which, for the sake of illustration, .is assumed to be .150 kc. The accuracy .of the instrument depends directly upon th'e stability of this oscillator, and it should, therefore, be suitably 'controlled by means of a piezo-electric element or the like. The oscillator output is applied-to thecontrol elementv of a counter 31 which may be a device similar to that described in British Patent 471,731, of December 4, 1935, which acteristics which are -indicated by the curves shown immediately adjacent the conductors which are connected to the three terminals.

The number 3 terminal, hereinafter described, provides a series of impulses whichoccur at, oneiifth the frequency of the oscillator 36, that is, the time between successive impulses is equal to $60,000 of a second. These impulses are applied to a dierentiator 39, the function of which is to decrease the duration of each impulse so that it will more accurately determine the operating time of a saw-tooth discharge tube which is represented by reference numeral 4I. The differentiator circuit is illustrated in Fig. 6 and is hereinafter described. The saw-tooth discharge tube is illustrated in Fig. '1, and provides a saw-tooth output voltage in the well-known manner. This voltage is applied to a phase splitter 43 illustrated in detail in fFig. 8 which provides as its output balanced saw-tooth voltages for energizing a push-pull deflection amplifier 45, which is a conventional amplier, the output of which' is connected between the horizontal deiiecting electrodes of a cathode ray tube 41.

The number 3 output terminal of the five-toone counter 31 is also connected to the control input circuit of a similar 10-to-1 counter indicated by reference numeral 49. The No. 2 output terminal of the counter 49 is a step voltage of the type indicated by the small curves adjacent connecting lead 5l which applies the output voltage ofthe counter 49 to an amplier 53. The amplifier is connected to one of the vertical deiiecting electrodes of the` cathode r y tube 41. As a result, a vertical deiiecting pote `tial is applied to the tube 41 which increases in ten equal steps, each step occurring at a time interval of 1/30,0n0 of a second. The complete cycle of ten steps is obviously accomplished in a time interval of /aooo of a second. As a result, the cathode ray beam is caused to move horizontally across the luminescent screen in 1/30,000 of a second at which time it is returned to its original starting point but one line lower, and the second line is then scanned. Alternatively, the vertical deflecting voltage may be derived from a saw-tooth discharge tube which produces a deilecting voltage similar to that applied to the horizontal deflecting electrodes, but in such a case the lines will not be horizontal but will slope downwardly, as is well known.

The No. 1 output terminal of the 5-to-1 counter 31 is connected to a phase reversing amplifier 55 the output of which is a unipotential voltage comprising a series of pulses separated by intervals of 1,650,000 of a second. The phase reversing amplifier is merely a conventional amplifier, and its purpose is to invert the negative impulses which are produced by the counter 31 so that they will have the proper polarity. The output of the amplifier 55 is connected by means of a switch 51 to the remaining vertical deilecting electrode of th'e cathode ray tube 41. As a result, small 10 marks or timing index pulses are produced along the ten scanning lines as indicated in Fig. l. Since these impulses are derived directly from the main oscillator 3B their timing is extremely accurate.

It has been pointed out above that a complete scanning cycle comprises ten complete frames each of which in turn contain ten horizontal scanning lines. It has also been pointed out that the rst transmitted impulse is derived from the first-timing pulse of the first line and the first frame. 'I'his impulse is indicated in Fig. 1 by the first impulse in the first line winch occurs at time To. It will be appreciated, therefore, that it will be necessary to select one particular pulse from the large number of pulses which occur during the period and use it to initiate the scanning cycle as well as to time the transmitted pulse. It will also be appreciated that but one pulse should be radiated during the period covered by the complete scanning cycle of ten frames. In

' order to accomplish this, the No. 3 output of the counter 49 is connected to the control input of a similar 10-to-1 counter 58. The No. 2 output terminal of the counter 58 then provides a step voltage of the type illustrated by the small diagram adjacent connecting lead 59. This voltage increases in ten equal steps from a given starting point and then suddenly returns to the original point at a time interval which is equal to 1/soo,ooo

K of a second. I'he 300 cycle step voltage which is so obtained is applied to the input of a differentiator and a clipper tube 6| the circuit of which is illustrated in Fig. 6. The function of this tube is to produce a positive impulse corresponding to the return time of the step voltage, the frequency of the impulses being, as a result,

300 cycles per second. The output of the differ-` entiator and clipper is connected to the No. 2 input terminal of a keying tube 63 which is illustrated in detail in Fig. 12. The phase reversing amplier 55 is connected to the No. 1 input of the keying tube 63. l

The function of the keying tube is to pass current only at such time as there is present on its input circuits a positive impulse from the phase reversing amplifier and a similar impulse from the dierentiator 6|. Since the 3 kc. counter 49 supplies the vertical deilecting voltage for the cathode ray tube 41, it will be recognized that the impulse derived from the step output of the 0.3 kc. counter 58 occurs at the instant the cathode ray beam has completed its entire scanning cycle of vten complete frames. Consequently, the resulting impulse which is produced by the diiferentiator GI-is concurrent with the No. l impulse of the scanning cycle. The circuit constants are adjusted so that the duration of the derived impulse is short enough to exclude the No. 2 input which occurs 1/15u,uou of a second later.

The output of the keying tube is a negative impulse having a small peak corresponding to the selected timing impulse. This impulse is applied to a reverser and clipper tube 65 the circuit of which is illustrated in detail in Fig. 14 The function of this tube is to eliminate the low frequency keying impulse and to obtain only the selected timing impulse as illustrated in the small curve adjacent the reverser and clipper tube 65. 'I'he output of this tube is then applied to a delay network 61 which may be, for example, of the series inductance-shunt capacitance type illustrated in Fig. 15. 'Ihe delay network is designed to have a total time shift at least equal to the time period between adjacent able, and is calibrated in terms of the total delay produced. By suitably adjusting the delay network, the time of transmission of the radiated impulse with respect to the time at which the scanning cycle is initiated is adjusted so that the calibrated time distance from the primary control station to the objective, based on the total path of transmission and the known constant delay, will coincide with one of the timing impulses in the desired frame which most nearly corresponds to the objective distance.

The output of the time delay network $1 is applied to an ultra high frequency transmitter 69 through a commutator device 1t. The transmitter itself is an impulse transmitter and may, for example, be of the type illustrated in Fig. 17. The commutator 1l may be mechanically or electrically operated, as desired, and preferably takes the form illustrated in Fig. 16. The function of the commutator 1l is to apply the selected control impulse to modulate the impulse transmitter 69 in alternation with other control impulses which are utilized in a manner which will hereinafter be described.

As indicated above, it is impractical to obtain the impulse which is to be used to key the transmitter to radiate a wave to the secondary control station by delaying the initial selected im-V pulse, since the required delay period would be excessive. It, therefore, becomes necessary to select another particular timing pulse `which may occur in any line of any of the ten frames. Assuming that it is desired to select impulse X from line t of the first frame, as indicated in Fig. 1, a selector system is required by means of which it is possible to select, rst, the first, second, third, fourth, or fifth timing impulse, the third .being selected in the present example, then to select the particular scanning line in which the desired impulse lies, that is, the fourth, and finally to select the oney of the ten frames in which the desired pulse occurs. The first portion of this selector system comprises `a pulse selector 13 the output of which is connected to the No. 2 output terminal of the 30 kc. counter 31. The circuit diagram of a suitable pulse selector is illustrated in Fig. 9, which will be later described. The function of the pulse selector is to derive a, short pulse whose duration includes the interval between successive steps of the 30 kc. step voltage produced by counter 31. The pulse selector is provided with a manual adjustment by means of which any one of the ve steps may be selected at will. 'I'he impulse which is so derived is passed through a limiter or clipper tube 15 the circuit diagram oi` which is illustrated in'liig.I 11. The function of this tube is to produce an essentially square positive impulse the time duration of which includes one of the ve equal steps of the 30 kc. voltage, and, consequently, it will be appreciated that this impulse coincides with every fifth timing impulse. However, this square impulse is not of sufficient accuracy to utilize by itself, so that it is necessary to combine it with a timing impulse from the phase reversing amplifier 55 in a keying tube 11 similar to that discussed previously. The output of the keying tube is applied to a phase reverser and Aclipper tube 19 which removes the control and passes only the selected timing impulse. There is thus obtained at the output of the reverser and clipper 19 an accurately timed short impulse which corresponds in time with every fifth timing impulse, and which may be made to coincide with the initiating impulse, or with any desired successive impulse by means of the pulse selector 13.

A similar system is utilized to select the desired line of the scanning frame; that is, a second line selector 8| is connected to the No. 2 or step voltage outputof the 3 kc. counter 49. The line selector 8l is connected to a third keying tube 83 through a clipper tube 85. The operation of these elements is identical to that of the corresponding elements described. immediately above. In this case, however, the line selector is adjusted to select the desired one of ten steps, rather than one of five, as in the preceding case. As before, the control impulse from the line selector is of insuiiicient accuracy to be utilized alone, so that it is combined with the selected pulse from the reverser and clipper 19 in keying tube 83 the output of which is again passed through a reverser and clipper 81 which removes the control impulse and permits only the accurately timed impulse to pass. By suitably adjusting the line selector 8i, it is, therefore, possible to obtain an impulse which corresponds in time to the selected impulse X but which is repeated during each of the ten scanning frames which constitute a complete scanning cycle. The nal step, consequently, is to select the desired frame period, and this is accomplished, as before, by a frame selector 89 which is coupled to the No. 2 output of the 0.3 kc. counter 58. The output of the frame selector is connected to a fourth keying tube 9| through a clipper tube 93, the output of the keying tube being connected, as before, to a reverser and clipper tube which in turn applies the selected pulse to a. delay network 91, 'I'he output of the delay network is applied to the commutator 1I and is, accordingly, applied to control the transmitter 69 along with the rst derived impulse from delay network 61 at alternate intervals.

The selector system so far described thus makes it possible to select any one of the 500 timing pulses which occur during each complete scanning cycle. Thus, it is possible to select the desired frame, the desired scanning line of the frame, and the particular pulse in the selected line. In practice, the device is adjusted to direct the aircraft to a particular objective by determining the time distance from the primary ground station to the objective, assuming that this distance is longer than the distance from the secondary control station to the objective. Assume that the time-distance from the primary ground station to the objective is less than indicated by the indexing pulse O of Fig. 1 and greater than the next preceding or 38th indexing pulse. I'he time-distance difference between the indexing pulse O and the actual time-distance is obtained by delaying the first transmitted pulse an amount suflicient to compensate for the said time-distance difference and for any retransmitted by the secondary ground station. 'I'he received pulses are indicated as P and S. As the aircraft or other vehicle is directed toward the objective, the indications P and S approach the index O. When the indications P and S coincide with the index O, the coincident indications show that aircraft should be at the objective.

It is apparent that some system must be provided for indicating which of the various pulses, lines and frames have in fact been selected. Such an indicator is provided by an auxiliary cathode ray tube 99 which may be a small 2-inch tube, or the like. The 300 cycle step wave output of the 0.3 kc. counter 50 is applied to the vertical deflecting electrode of the auxiliary cathode ray tube 99. The 3000 cycle step wave output of the 3.0 kc. counter 49 is applied to the horizontal deecting electrode. Consequently, a series of ten lines is produced similar to the ten lines of the main cathode ray tube 41. The No. 2 output of the 0.3 kc. counter 58 is also applied to another frame selector which is identical to those previously described. Its output is connected, as usual, through a Clipper tube |03 which in turn is coupled to the control grid |05 of the cathode ray tube 41. This 'control grid is normally given a negative bias so that the cathode ray beam is extinguished until such time as a positive impulse is applied from the output of the clipper tube |03. The time duration of this impulse corresponds to the time period required to complete one. scanning frame; that is, 1fooo of a second, Consequently, the cathode ray screen remains dark during a period corresponding to nine scanning frames.

The output of the clipper |03 is also applied to a phase reverser |01 which in turn is coupled to the control grid of the auxiliary cathode ray tube 99. 'Ihis control grid is normally biased so that the cathode ray beam is operative, but, due to the phase reverser |01 when an impulse is applied to the main cathode ray tube to produce a frame, the same impulse is applied to the auxiliary cathode ray tube to extinguish it for a corresponding period. Now the vertical scanning period is l/oo of a second. If an impulse of M4000 of a second is applied to bias off the tube, it is apparent that one of the ten lines will be removed. It will, therefore, be an easy matter to count the remaining lines in order to determine which line has been selected. The number of the line selected, as indicated on the auxiliary cathode ray tube, corresponds to the frame which is applied to the main cathode ray tube 41; that is, if an observation of the auxiliary cathode ray tube indicates that the third line is missing, it may be established that the frame selector |0| has been adjusted to select the third frame. The operation of the line and pulse selectors may be checked by utilizing switch 51 which in its check position" connects the nally selected keying impulse from the output of the reverser and clipper 95 to one of the vertical defiecting electrodes, while removing the superimposed timing impulses. It is apparent that the position of the selected impulse should coincide with the position of one of the timing impulses on the cathode ray screen. Thus, by operating the switch 51, noting the position of the single selected impulse which will appear in the check position, and returning the switch to its normal position, the line and number of the selected pulse may be determined.

The two receivers |09 and I are also included in the aircraft equipment. These receivers may be separate or they may have a common input and antenna system broadly tuned to include signals from both of the control station transmitters. The output of the first receiver |09 is connected through a phase reverser 3 to a commutator 5. The output of the second receiver is connected directly to the commutator I5. The commutator is synchronously operated from a commutator control voltage source |1 with the commutator 1|. lAs a result, the outputs of the rst and second receivers are alternately connected to one of the vertical deflecting plates of the cathode ray tube 41. As pointed out above, this permits the signals from the two ground stations to be alternately received. The phase reverser ||3 inverts the phase of the voltage from one of the receivers so that the variable indexing impulse S, for example, may be distinguished from the variable indexing impulse P.

I have thus described the general system by means of which a preferred embodiment of my invention may be carried into practice. As to the specific details of the various elements which have been schematically indicated in Fig. 3, it is to be understood that there are many diierent systems which may be utilized. However, in order to fully describe an operative system, an illustrative embodiment of each of the elements which are not well known to those skilled in the art will now be described.

Referring to Figs. 4 and 5, I have illustrated a frequency counter of the type described in the British Patent 471,731, previously mentioned. Sine wave input is applied through a limiting resistor to the grid of a tube ||9, the plate circuit of which is energized from a suitable source of D.C, voltage through a resistor |2| and a peaking inductor |23- The plate is also coupled through a small capacitor |25 to the cathode of a diode rectifier |21 and the anode of a second diode rectifier |29.

The anode of the rst diode rectier |21 is connected to output terminal No. l and is also connected to ground through a resistor |3I. The cathode of diode |29 is connected to ground through a large capacitor |33, to the grid of a cathode follower tube |35, and through an in ductor |31 to the grid of an output tube |39. Output terminal No. 2 of the frequency counter is connected to the cathode of the cathode follower tube |35. Output terminal No. 3 is connected to the anode of the output tube |39. The output circuit of this tube also includes an inductor |4| which is coupled so as to produce a regenerative feedback to the grid circuit inductor |31 A positive bias is applied to the cathode of the output tube |39 by means of a voltage divider |43.

Input tube I I9 is normally drawing plate current. As its grid becomes negative during each cycle of applied voltage, its plate voltage becomes more positive, thus causing a small constant charging current to flow through capacitor |25 and diode |29 into capacitor |33. The 'capacity of capacitor |25 is quite small with respect to that of the capacitor |33 so that the total charge received by the latter capacitor is not sulcient to charge the capacitor up to its full capacity. The voltage across the large capacitor |33 consequently takes the form of the step wave illustrated in Fig. 5. A similar output voltage is then applied to output terminal No. 2 by means of the cathode follower tube |35.

When the applied voltage causes the grid of the input tube to become positive, the plate voltage decreases, as is well known. This causes a *vis l' ne ative ulse t`o be applied to the cathodeoi' the diogde lz'lpand current vflows through resistor |3|, producinga negative pulse at output terminal No. 2, lasjillu'strated in `lilg. 5.. 'A similar negative pulse is produced each time the grid exceedsl the cut-oif-value of the input tube.. However, durln'g this interval. v,capacitor m l maintains its` chargasince it is now yisolated by rectifier |25.

As a-result,ea.ch reversal of the applied voltage causes the charge in the capacitor |33 to be in- -reased until ilnally at some predeterminedvoltz ge .the negative'bias which is normally applied to the output tube |39l is overcome. l When this occurs, the beginning plate current' produces-a regenerative feedback voltage on the grid which the` output tube rapidly becomes more negative and control by t,he xed negative bias -is reestab-"Q lished. The point at which the condenserA i3d-'is discharged is determined by the adjustment volf` '1 the voltage divider |43.

To operate as a 5to1 frequency counter, the

fixed bias applied to output tube is 'adjusted to a value suihcient to cause the-tube'to become' triggered at thefth step, as indicatedin Fig. 5. In order to operate as a 10to1 frequency counter, it is merely necessary to increase the ixed bias which is `applied to the output Ltube so that it will betriggered into action when the tenth charging impulse is applied to capacitor |33.

Fig. 6 is a diil'erentiator whichis merely an `amplier in which the input capacitance and grid resistance are so selected thatthe amplifieratube |45 responds to rapid changes in the potential of the applied voltage. In order to prevent thehigh frequency components of the applied .pulses from being attenuated, the plate circuit of the amplifier includes a compensating ret/ttorY |41 nected inshunt with aresistor |49.y

The saw-tooth discharge tube illustrated vfv'in 'l Fig. 7 includes fai capacitance-resistance*input coupling system Aand a thermio'nic'tube v|5|,"thel plate circuit of'which is energized by a source 'of D.-C. potential through a high resistance |55.

A small capacitor |55 is connected across the platey circuit resistor |53.V -In operation, the' capacitor |55 is slowly charged through resistor |53`at a rate which is`substantially'linear over a small range. by the applied impulses, the anode-cathode iinpedance of the tube |5| becomes very low-and 'I'he saw-tooth out'-A put voltage which is produced is illustrated in the small curve directly above the output terminal;V '9

discharges the condenser.`

Fig. 8 illustrates a phase splitter, and comprises le tnermiometube |51 the output voltage `of which l y is divided between the, anode and `cathode ciiitsiizhat is, resistors |59 andV IBI of'equal-fiin pedance are connected in the cathode and al'iod'e circuits of the tube, respectively; balanced output being taken from the anode and cathode elecf',

trodes.

The pulse selector illustrated in Fig. 9 comprises 'Y a thermionic tube |83 which is provided with-a potentiometer |65 for applying an adjustable f positive voltage to its cathod A series limiting f Fig. 11.

conv ltube, f0r example, is a vtulle |15 of the 6L7" type having a pair of grids which are'connected re'` Spectively to the No. 1 input and the No. 2 input I terminals. The output of the keying tube is. de-

Aty predetermined intervals, determined u amas@ .f 'Y

resistor lll may be included in the grid; circuit. Output is taken 'from a capacitor I coupled tof its anode. -Q'Ihe amplitude of the applied step voltage is selected so that the increase for anyv given step exceeds the voltage necessary to cause Y the tube to pass from plate current cut-oir. to-

zerobias, as is indicated in Fig. 10;V that is, the

cut-0l! point fOr the tube is determined by-d- Justment of the potentiometer |55.v lThis value is set to slightlvexceed the voltageffor j any given step, s'o that, whenthe 4voltage rises for. the nextsucceeding step,.the tube will draw Y a.l large plate currentvand its plate vvoltage will 'rapidly decreaseto a v It; will be appreciated that. by adjusting thepof 'tentiometer |65, -plate .current .may be .l j'ilow at a time' periQdcorresponding' to`any de-` 1 one of theseveralsteps.

v v' I'he outputvoltagel of the pulse selector is also ill y. pulse selector is applied to a tube |13. y The amf- 1 plitude of the applied pulse is preferably sumvcient to causethe grid voltage to exceed the cut oil value. l As a result, the peak amplitude of the? 1 output voltage is limited in a manner illustrated in the small curve above the output terminal'ofw'' At the same time, the polarityvof thel vpulse is inverted, 'so thatthe output of the clipper" tube comprises a positive pulse of substantially rectangular form which starts at a time ccrre- -sponding to the increase in voltage at ,theselected step and whose duration is'suiilcient to include a E;

period up to but exclusive of the next succeeding step.

to which reference is now made.'A 'Ihek rived-from the plate circuit which is' energized through a series impedance.- In the absence of an applied grid voltage, the tube isbiasedjte a substantially low valueof plate current. Short'` impulses of the type produced by the No. 1 out.

put ofthe counter tubes are applied to the No. .1 arid.

coupled amplifier a. biased ampliiler tube |18 ofthe cathode folbstenceuylewr v elaej..

. @trawl-1n 10, and is lscenic be adifferentlal voltage Y. -which becomes suddenly negative whenthe plate 1 current increases. and the plate voltage decr. .and then slowly rises again to its steady state. j The output of the pulse selectoris appliedto -a. clipper-- tube which'is illustrated in Fig. 1 1. lThis tube includes a grid resistor-"Illv throughwhich the negative pulse produced by the' Referring for a moment to Fig. 3 itwill be Y noted that the output of the clipper tube is ap- Y vplied to a keying tube which has two input ter'A l the remaining terminal beingenergizedby an indexing impulse corresponding tothe original oscillator frequency.

v illustrated in Fig. 12, and the applied voltages f 1 are indicated in the sketch illustrated vin Fig. 1 3" Such a keying tubeis The substantially rectangularv impulsev from theclipper. tube applied to .the No. 24. grid. 'I 'he resultant output voltage illustrated in Fig. 13 is a combination of the two input volti vages and comprisesfa substantially rectangular.v negative outputimpulse havinga small negative ,'-peak'at its 'minimum point.. This voltage isthen. applied'to a phase reverser and clipper such asl that illustrated in Fig. Maand to" which refer-#- f i v enceisnowmada Y l A The' phase reversing section of this device in-v vcludes al capacity-resistance tube |11, the plate circuit of whichis .coupled to A potentiometer |83.

across a cathode resistor IBI, and a xed positive bias is applied to the cathode by means of a potentiometer |83. As a result, the combined keying impulse and timing impulse A produced by the keying tube is inverted by the first amplifier tube |11 to produce a wave of the type illustrated at B in the small diagram of Fig. 14. The horizontal section of the keying impulse B corresponds to the cut-oi bias of the output tube |19. This is accomplished by a suitably adjusted As a result, all the timing impulses, except that one which coincides with the keying impulse, are eliminated by the rst reverser and clipper. In addition, the keying impulse itself is eliminated, since the output of the clipper tube responds only to potentials in excess of the peak potential of the keying voltage. As a result, a single impulse C is selected which corresponds in time with the impulse produced by the pulse selector.

Fig. 15 represents an adjustable time delay network suitable for use'in connection' with this invention and comprises a series of inductors L and capacitors C connected in the well-known manner. The operation of this network is conventional and need not be described in detail.

Fig. 16 represents an electronic commutator suitable for use in connection with this invention. The commutator includes a pair of double grid thermionic tubes, of the 6L? type, for example, |85 and |81. Input No. 1, from the delay network 6l of Fig. 3, for example, is coupled to one grid of the first tube |85. Input No. 2, from the delay network Sl of Fig. 3, for example, is connected to the corresponding grid of the second tube |87. An alternating voltage is applied in phase opposition from a suitable source of A.C. voltage through a switch |88 in the remaining grids of the two tubes. The output anodes are connected in parallel and suitably energized by a source of positive potential. Operating the switch |88 removes the A.C. control voltage so that both tubes are simultaneously operative, and the commutating action suspended.

In operation, the applied A.C. control voltage causes the two tubes to become alternatively conductive s`o that the voltages applied to the respective input circuits are alternately amplified and connected to the output terminal.

Fig, 17 illustrates an impulse transmitter of the type suitable for use 'in connection with this invention. Since the radiated pulses are of extremely short duration, it is possible to operate the tube at a severe overload for such a short interval. A small tube may be utilized to radiate a pulse of large amplitude, since the tube is actuallyin operation only an extremely small portion of the total time. Consequently, a voltage supply source |89 is provided which supplies, for example, 10,000 volts at 1 m. a. This voltage is applied to the anode of an oscillator tube |9| which is connected in a conventional oscillator circuit. However, the oscillator grid is normally biased to cut-oir so that the tube is inactive until such time as a suitable positive pulse is applied to its grid through input terminal |93. As a result, a short impulse of large amplitude is radiated from the antenna |95.

I claim as my invention:

1. The method of directing a movable object to a predetermined objective which ls a known distance from two control stations which are at known locations, which comprises radiating pulses of radio energy from said movable object,

receiving and reradiating said pulses at said control stations, receiving said reradiated pulses at said object, producing a rst indication corresponding to the time period required for a pulse to travel from said predetermined objective to one of said control stations and back, producing a second indication corresponding to the time period required for a pulse to travel from said movable object to the other of said control stations and back, and comparing said rst and second indications with a distance indication which is a time function of said known distance to determine when said object has reached said predetermined objective.

2. In the guiding of a movable object to a predetermined point which is a known distance from ,Y two control stations having Vknown locations, the

method which comprises radiating pulses of radio energy from said movable object, receiving and reradiating said pulses at said control stations, receiving said reradiated pulses at said object, producing one indication corresponding to the time period required for a pulse to travel from said predetermined objective to one of said control stations and back, producing another indication corresponding to the time period required for a pulse to travel from said movable object to the other of said control stations and back, producing as a function of time a third indication corresponding to said known distance, and utilizing said iirst and second indications by comparison with said third indication to determine when said object has reached said predetermined point.

3. In the guiding of a movable object to a predetermined point which is a known distance from a control station at a known location, the method which includes radiating successive short pulses of radio frequency energy from said movable object, receiving and reradiating said pulses at said control station, receiving said reradiated pulses at said object, the time length of said pulses being small with respect to the time required for each pulse to travel from and to said object, and the interval between successive pulses being greater than said time so that each pulse is radiated and received before the next succeeding pulse is radiated, producing as a function of time one indication corresponding to the calculated time period required for a pulse to travel from said predetermined point to said control station and back, producing a second indication corresponding to the time period required for a pu1se to travel from said movable object to said control station and back, the relative displacement of said first and second indications being a measure of the distance of said object from said predetermined objective.

4. In the guiding of a movable object to a predetermined point which is a known distance from two control stations at known locations, the method which includes radiating successive short pulses of radio frequency energy from said movable object, receiving and reradiating said pulses at said control stations, receiving said reradiated pulses at said object, the time length of said pulses being small with respect to the time required for each pulse to travel from and to said object, and the interval between successive pulses being greater than said time so that each pulse is radiated and received before the next succeeding pulse is radiated, producing as a function of time an objective index corresponding to the time period required for a pulse to travel from said predetermined objective to the most distant control 19 station, timing the transmission of the second of said successive radiated pulses with respect to the rst radiated pulse to compensate for diiIerences in distance between said objective and said two control stations, and utilizing said received reradiated pulses to produce position indexes corresponding to the time periods required for said pulses to travel from said object to respective control stations and back, the relative displacement of said objective index and said position indexes being a measure of the distance or said object from its objective.

5. The method of indicating the position of a movable object with respect to a predetermined objective the location of which is known with respect to the locations of a pair of control stations which comprises, radiating a, first series of pulses of radio frequency energy from said object, receiving and reradiating said pulses at the control station furthest from said objective, radiating alternately pulses of a second series, receiving and reradiating said second series of pulses from the control station nearest said objective, the time period between corresponding pulses of said iirst and second series being equal to the difference in the time required for the pulses of said iirst and second series to travel from said objective to respective control stations and back, receiving said reradiated pulses at said object, producing an objective index corresponding to the time period required for a pulse to travel from said predetermined objective to said furthest control station, and utilizing said received pulses to produce position indexes corresponding to the position of the said object with respect to said control stations.

6. The method of indicating the position of a movable object with respect to a predeterminedl objective the location of which isv known with respect to the location of a pair f contll Stations which comprises radiating a rst series of pulses of radio frequency energy from said object, receiving and reradiating said pulses at the contro1 station furthest from said objective, radiating alternately pulses of a second series, receiving and reradiating said second series of pulses from the contro1 station nearest said objective, the time period between corresponding pulses of said rst and second series being equal to the diiference in the time required for the pulses of said first and'second series to travel from said objective to respective contro1 stationsv and back, receiving said reradiated pulses at said object, producing a. cathode ray beam, deiiecting said beam to produce a known time trace, producing on said trace an objective index, the position of which corresponds to the time distance from said objective to said furthest station and back, and utilizing said received pulses to produce position indexes on said trace, the position of said indexes corresponding to the instantaneous position of said object with respect to said control stations.

7. In the navigation of a movable object to a predetermined objective which is a known distance from two contro1 stations. the method which includes radiating a succession of discrete pulses of radio frequency energy from said object, 'receiving and reradiating said pulses at said control stations, receiving said reradiated pulses at said object, establishing electrically in accordance with a time function an objective index correspending to the calculated time for a pulse to` travel from said objective to one of said contro1 stations, and utilizing said received pulses to produce indications of the instantaneous time dis- 75`10 20 tance of said object from said control stations, whereby the distance of said object from s ebjective may be determined.

8. In a system for directing a movable object to a predetermined objective which is located at known distances from two control stations the combination of means for radiating a succession of pulses of radio frequency energy from said object, means located at each of said control stations for receiving and reradiating said pulses, means for separately receiving at said object the pulses reradiated from said two control stations, a cathode ray tube eiectively connected to said receiving means and including ray deflecting means for producing a time measuring trace, and means including said cathode ray tube for measuring the time required for pulses transmitted from said object to 'be reradiated and returned to said object, and means for producing an objective index on said cathode ray trace, said index corresponding to the time interval required for a pulse to travel from the location of said objective to one of said control stations and back.

9. A system for indicating the position of a movable object with respect to a predetermined objective, said objective being a known distance from two contro1 stations which'includes means for radiating a pulse of radio frequency energy from said object, means located at said control stations for receiving and reradiating said pulse, means for separately receiving said reradiated pulses. time measuring means, means for producing predetermined timing indications on said time measuring means correspondingl to the times required for a pulse to travel from Athe location of said objective to said control stations and back. and means for producing indications on said time measuring means corresponding to the time intervals required for a pulse to travel from said object to said contro1 stations and back, the difference between corresponding indications being a measure of the distance of said object from its objective.

10. A system for directing a movable object to a predetermined objective which is located at known distances from a pair of control stations which includes in combination means for radiating a first series of pulses from said object, means for receiving and reradiating said pulses from one oi.'v said control stations. said rst series of reradiated pulses at said object, means for radiating a second series of pulses from said object, the pulses of said rst and second series being radiated alternately, means for receiving and reradiating the pulses of said second lseries at the other of said control stations, means for receiving said second series of reradiated pulses -at said object, indicating means kresponsive to said reradiated pulses and including said receiving means for producing position index marks corresponding to the times saldreradiated pulses return to said object, and means for applying as a junction of time an objective index mark on said indicating means, the arrival of said object at the location of said objective being determined by the coincidence of said objective mark and said position index marks.

11. A device of the character described in claim 10 in which the time period between corresponding pulses of said rst and second series of pulses is equal to the difference in the time required for a pulse to travel from said objective to respective control stations and backto said objective.

12. A device of the -character described in claim means for receiving t 21 cathode ray tube having beam deiiecting means for producing a line trace, the position of said index mark along said line trace being a measure of the relative time sequence of said marks.

13. A device of the character described in claim 10 in which said indicating means comprises a cathode ray tube having beam deecting means for producing a plurality of separate linear traces, said traces being drawn in time sequence, the position of said index marks along said line traces being a measure of the time sequence of the reception of said reradiated pulses.

14. In a device of the character described, a movable object, means for radiating a succession of pulses of radio energy from said object, at least two control stations Whose locations are known, means for receiving and reradiating from said control stations distinctive pulses received from said object, a cathode ray tube mounted in said object, scanning means for producing line traces on said cathode ray tube, and means connected to said cathode ray tube and separately responsive to the reception of said distinctive reradiated pulses for producing index marks on said traces, the positions of said index marks along said lines being measures of the transit time of said pulses.

15. A device of the character described in claim 14 which includes, in addition, means for Vproducing as a function of time an Aobjective index on one of said line traces, the position of said objective index being indicative of the destination rof said object. 

